During the assembly of semiconductor packages, semiconductor chips are often attached onto carriers, such as substrates or leadframes, for processing multiple semiconductor chips at the same time. After or during attachment, electrical connections are made between electrical pads on the chips to corresponding contacts or connection pads on the substrates or leadframes. This can be done by wire bonding, or the electrical pads can be directly attached onto the contacts on the substrates or leadframes. Thereafter, it is usually necessary to protect the chips and the electrical connections from the environment by encapsulating them in a molding compound, such as epoxy molding compound (“EMC”).
In a typical transfer molding process, a substrate with the chips attached is placed into a molding system comprising top and bottom molding halves and sometimes, a middle plate. One or more molding cavities are formed in one or both of the mold halves corresponding to the positions of the chips to be encapsulated. Molding compound is introduced into mold supply pots in the molding system. The mold supply pots are linked to the molding cavities through a system of runners and gates through which the molding compound is channeled before entering the molding cavities. A plunger is insertable into each pot and the molding compound is distributed from the mold supply pot by the plunger compressing the molding compound.
After the cavities have been filled, the molding compound is allowed to set and harden. Besides molding compound that is filled into the cavities for encapsulation, excess molding compound is also created inside the mold supply pot, and in the runners and gates. The excess molding compound can be conveniently referred to as cull, runner portion and gate portion. These need to be removed and discarded before further processing of the leadframe.
FIGS. 1A to 1D are schematic cross-sectional views of a conventional molding apparatus 10′ illustrating a package pin 12′ for forming a fiducial mark on a molded package during a molding process. The package pin 12′ is mounted to a top mold chase 14′ of the molding apparatus 10′, which is movable relative to a bottom mold chase 16′ and a middle plate 18′. The package pin 12′ contacts a surface of a molded package 20′ mounted on a substrate 19′, and creates fiducial markings on the surface for orientating the molded package 20′. The package pin 12′ is located in the top mold chase 14′ and extends from the top mold chase 14′ through the middle plate 18′ when the top mold chase 14′ and the bottom mold chase 16′ are closed. In this position, the package pin 12′ slightly contacts a surface of the molded package 20′ positioned on the bottom mold chase 16′ to form a small dot or depression on the surface of the molded package 20′. During removal of the cull, runner portion and gate portion, the top mold chase 14′ is separated from the middle plate 18′ as shown in FIG. 1B. The package pin 12′ located in the molding apparatus 10′ is pulled away from the surface of the molded package 20′ together with the top mold chase 14′ in this process.
After the cull, runner portion and gate portion are removed, the top mold chase 14′ closes onto the middle plate 18′ and the package pin 12′ contacts the surface of the molded package 20′ again as in FIG. 1C. FIG. 1D shows the separation of the middle plate 18′ together with the package pin 12′ from the bottom mold chase 16′ during removal of the molded package 20′. At the time when the top mold chase 14′ closes onto the middle plate 18′, the package pin 12′ may create a second marking on the molded package 20′ which overlaps the first marking so that the initial dot becomes less distinctive on the surface of the molded package 20′. It is therefore desirable to devise a method which avoids the package pin 12′ contacting the molded package 20′ a second time, in order to produce sharp fiducial markings on the surface of the molded package 20′.
As regards another aspect of the molding apparatus 10′, expulsion of molded packages requires ejection pins to push molded packages away from the middle plate 18′ after molding. FIGS. 2A to 2C are schematic cross-sectional views of a conventional molding apparatus 10′ illustrating ejection pins 22′ for pushing away molded packages after molding. The ejection pins 22′ are mounted at one end to an ejection plate 24′, and are guided through loose-fitting holes of a runner plate 26′ towards the middle plate 18′. The ejection plate 24′ is located on the molding apparatus 10′ such that the ejection pins 22′ extend through the top mold chase 14′ and the runner plate 26′, and then project through receiving holes 28′ located in the middle plate 18′. After molding, the ejection pins 22′ are operative to press and push out molded packages 20′ mounted on a substrate 19′ on the bottom mold chase 16′ through the receiving holes 28′ when the molding apparatus 10′ is opened (see FIGS. 2B and 2C).
Generally, there is a relatively large clearance between the receiving holes 28′ and the ejection pins 22′. Otherwise, the ejection pins 22′ which are guided loosely by the runner plate 26′ may be misaligned with the receiving holes 28′ and hit the middle plate 18′ when the molding apparatus 10′ is being closed. This may damage the middle plate 18′. However, a substantial amount of molding compound is wasted when the molding compound enters and remains in the relatively large receiving holes 28′. Cleaning of an elastic film lining the bottom mold chase 16′ would also require substantial effort because the film tends to slip into the relatively large holes 28′. It would therefore be useful to devise a way of ejecting the molded packages 20′ without the excessive loss of molded compound and without having to form receiving holes 28′ which are overly large and without damaging the middle plate 18′ during molding.